A nitride semiconductor, as an active material for an electronic device such as the field-effect transistor and the likes and for a light-emitting device in a short-length region from a visible light region to an ultraviolet light region, has been actively studied and technically developed in recent years.
In general, a nitride semiconductor is formed on a substrate comprising SiC, Si and the likes. A Si substrate in particular has a numerous advantages such as having availability for a large dimension thereof at a low cost, an excellent crystalline property and radiation performance, as well as easy cleavage and etching and a mature process technique.
However, since a lattice constant and a coefficient of thermal expansion are significantly different between the nitride semiconductor and the Si substrate, there has been a problem that, if the nitride semiconductor is grown on the Si substrate, a grown-up nitride semiconductor causes the crack and the pit (a point-like defect). Since the crack and the pit result in a leak current which seriously affects a device property, it is an important issue to prevent occurrence thereof.
As a method to solve the above problem, there is known an art to prevent the crack by forming a buffer layer between the Si substrate and a nitride semiconductor layer. As disclosed in Patent Document 1, for example, there is the semiconductor material in which an interlayer comprising the nitride semiconductor is provided on the Si substrate and a buffer layer comprising compositionally graded AlXGa1-XN and the likes, and gallium nitride is formed on a transition layer.
As another method to solve the above problem, there is known an art such as, as disclosed in Patent Document 2, a nitride semiconductor device in which an AlN-based superlattice composite layer comprising a plurality of a high Al-containing layer and a low Al-containing layer laminated alternately is formed on the Si substrate and the nitride semiconductor layer is formed on the buffer layer, the AlN-based superlattice composite layer.
However, although being effective in preventing the clack generated in the nitride semiconductor layer, the semiconductor materials alone described in Patent Document 1 and Patent Document 2 do not have sufficient effect to prevent generation of the pit and are incapable of preventing degradation of the device property caused by generation of the pit. In addition, when the semiconductor material is used for a power device usage, the nitride semiconductor layer needs to have a thick film, as a high pressure resistance is required. Therefore, it is desired to improve the crystalline property of the nitride semiconductor, although the semiconductor material in Patent Document 2 does not have an effect to improve the crystalline property.    Patent Document 1: Japanese Patent Application Laid-Open No. 2004-524250    Patent Document 2: Japanese Patent Application Laid-Open No. 2007-67077